for my $anonhash ( @AoH ) {
    my $newkey = join '_', 'a', $$anonhash{id_a}, 'b', $$anonhash{id_b};
# one way (HoH):
    $master_hash{$newkey}{value_x} += $$anonhash{value_x};
    $master_hash{$newkey}{value_y} += $$anonhash{value_y};
# another way (HoA):
    $master_hash{$newkey}[0] += $$anonhash{value_x};
    $master_hash{$newkey}[1] += $$anonhash{value_y};
}